Modernize a few things

[Adafede]

Hi @EBjerrum, @JanoschMenke,

First, thank you so much for this tool!

Because when I tried to install and run it many old dependencies could not be found, I made a very humble attempt to modernize a few things:

• [x] Switching from setup.py to pyproject.toml (allows installation using pdm, poetry, and consorts).
• [ ] Switching from PySide2to PySide6.
  • [ ] I had to disable some buttons/shortcuts I was not able to fully port to PySide6. All the parts are commented by ## TODO (AR): I could not getting this to work with PySide6

I believe these are easily fixable, but error messages were not very informative, except PySide6seems more stringent regarding the classes.

I hope this helps some ways.